1. Field of the Invention
This invention relates to a process for producing highly branched polyisoolefines by the reaction of isoolefines, optionally in combination with conjugated or non-conjugated dienes and/or cationically polymerisable, singly- or multiply-unsaturated compounds, in a single stage process in solution, suspension or in the gas phase.
2. Description of the Prior Art
The cationic polymerisation of isoolefines has long been known, and is described by J. P. Kennedy and B. Ivan in Carbocationic Macromolecular Engineering (1991), for example. On account of their physical properties, the butyl rubbers and halobutyl rubbers produced in this manner are used in industry, particularly for the production of tyre tubes and inner liners for tyres. In this connection, the processing properties of the butyl rubbers produced in this manner during compounding, rolling, extrusion and calendering are particularly important. The processing properties are associated in particular with a balanced ratio of the green strength of the rubber and to the stress relaxation thereof. This can be achieved, for example, by blending corresponding polymers with different molecular weights to form products having a custom-made, broad molecular weight distribution. This process is laborious, however. The direct synthesis of butyl rubbers which exhibit a broad molecular weight distribution and random long-chain branching and which have the desired processing properties can be accomplished, for example, by the copolymerization of isobutene with isoprene in the presence of bifunctional monomers such as divinylbenzene, 2,5-hexadiene or vinylbenzyl chloride. However, one significant disadvantage of this copolymerisation is the formation of high proportions of gels in the rubber (see H. -C. Wang, K. W. Powers, J. V. Fusco, ACS Meeting, May 1989 Paper No. 21, for example).
Another method of introducing long chain branching occurrences was therefore introduced, namely the copolymerisation of isobutene and isoprene in the presence of multi-functional branching agents. The latter are to be understood as soluble polymers which contain functional groups, and which under the process conditions either initiate polymerisation ("grafting from" by tertiary alkyl groups) or react with the cationic end of the growing polymer chain ("grafting onto" by reactive double bonds). Hydrochlorinated poly(styrene-coisoprenes), chlorinated polystyrenes, polyisoprenes or styrene-butadiene block copolymers have been mentioned as multi-functional branching agents (H. C. Wang, K. W. Powers, J. V. Fusco, ACS Meeting, 9-12.05.1989; EP-A 320 263). The resulting polymer mixtures are termed "star branched butyls". A disadvantageous of this procedure is the necessity for separate, additional process steps for the polymerisation or halogenation of the branching agents. The simultaneous formation of linear and branched polymers during polymerisation is a characteristic of this procedure.
The production of "multi-arm star" polyisobutenes by the reaction of active polyisobutene polymers with divinylbenzene is described in Polymer Bull. 31 (1993) 665. These polyisobutenes are produced by the "arm-first, core-last" method, which is explained in U.S. Pat. No. 5,458,796. Another possibility for the production of branched butyl rubbers is the use of multi-functional initiators, as described in U.S. Pat. No. 5,084,522. This method is also termed the "core-first, arm-last" method. This procedure is also burdened with some disadvantages, on account of the separate process steps for the production of the multi-functional initiator and for the formation of homopolymers by transfer reactions.